the boat should be idling away from you. Just before the line goes tight lean back against the rope to take out that last little bit of slack. When the boats starts it's initial acceleration jump off the dock and off you go.

To get the timing right, try sittin on the edge of the dock with the handle in your hand and have the boat pull you up that way.

In addition....make sure you jump and don't just let the boat pull/slide you off the dock....I had a guy strip a fin right of my board that way. You may also want to get an old piece of carpet to throw down if you don't want to scratch up your board......

Some people also reach out an arms length of rope and hold the rope in one of their hands. When the slack is used up to the point the rope becomes tight in your hand holding the rope they yell their favorite "hit it" term and off you go.....